Power converters are used to convert alternating current (AC) power to suitable direct current (DC) levels for powering an apparatus, such as laptop computers and other appliances. It is necessary to control the output voltage and/or current of a power converter, particularly under variable load conditions.
Prior art techniques for controlling series-resonant converters include variable frequency control, phase-control and fixed frequency pulse width modulation (PWM) duty-cycle control. These control methods directly affect the operating state or mode of the resonant tank in the power converter by altering the operating frequency or conduction intervals of the switching elements. Consequently, the resonant tank current and voltage phase angle relationships are altered in order to realize closed-loop output control.
A significant disadvantage of these control methods is that they achieve regulation at the expense of losses caused by the switching regime. What is needed is a control circuit and method that allows the converter to maintain a constant switching frequency and nearly load independent resonant tank characteristics across an entire load variation range.